The present invention relates to a two-component dry-type developer for developing latent electrostatic images to visible images, for use in electrophotography, electrostatic recording methods and electrostatic printing methods.
Conventionally, as a developer for use in these fields, the so-called two-component dry-type developer is known, which comprises carrier particles and toner particles. In the two-component dry-type developer, the particle size of the toner particles is very much smaller than the particle size of the carrier particles, and the toner particles are triboelectrically attracted to the carrier particles and are in fact held on the surface of the carrier particles. That electric attraction between the toner particles and the carrier particles is caused by the friction between the toner particles and the carrier particles. When the toner particles held on the carrier particles are brought near or into contact with a latent electrostatic image, the electric field of the latent electrostatic image works on the toner particles to separate the toner particles from the carrier particles overcoming the bonding of the toner particles to the carrier particles, with the result that the toner particles are attracted towards the latent electrostatic image and the latent electrostatic image is thus developed to a visible image. In the case of the two-component dry-type developer, only the toner particles are consumed as the development is performed. Therefore, it is necessary to replenish the toner particles in the course of repeated development by the apparatus.
Further, it is necessary to triboelectrically charge the carrier particles to the desired polarity and with a sufficient amount of charges, and to maintain the amount of the charges and the polarity for a long period of time during use. In the case of a conventional two-component type developer, it is apt to occur that the resin contained in and released from the toner particles in the course of the mechanical mixing of the toner particles and the carrier particles in the development apparatus, eventually covers the surface of the carrier particles. Once the surface of the carrier particles is covered with the resin, which is generally referred to as the "spent phenomenon," such carrier particles no longer function as active carrier particles, that is, carrier particles which contribute to development. As a result, the charging characteristics of the carrier particles in the conventional two-component type developer deteriorate with time while in use. In the end, it is necessary to replace the entire developer by new developer.
In order to prevent the spent phenomenon, a method of coating the surface of carrier particles with any of several different resins has been proposed.
For instance, in the case of carrier particles coated with tetrafluoroethylene fluorocarbon polymers, the spent phenomenon occurs only slightly due to the low surface energy thereof. However, since tetrafluoroethylene fluorocarbon polymers are on the extreme negative side in terms of the triboelectric charging series, carrier particles coated with tetrafluoroethylene fluorocarbon polymers cannot be used with toner particles which are to be charged to a negative polarity.
Further, in the case of carrier particles coated with silicone resin, which also has a low surface energy, silicone resin, too, is considerably on the negative side in terms of the triboelectric charging series (although not as extremely so as in the case of tetrafluoroethylene fluorocarbon polymers). Therefore, it has never been considered that carrier particles coated with silicone resin could be used for charging toner particles to a negative polarity.
As for toner particles, many negative polarity control agents for making toner particles easily chargeable to a negative polarity by addition thereof to the toner particles have been proposed, for instance, in Japanese Laid-Open Patent Applications Nos. 49-21151, 49-27229 and 52-67331. However, the negative polarity control agents disclosed in those Japanese Laid-Open Patent Applications are effective to control the charging polarity of non-coated carrier particles, but they are not effective for practical use to control the charging polarity of toner particles which are triboelectrically charged to a negative polarity by friction with carriers coated with silicone resin. This is because, of those polarity control agents, some are effective for initial charging of toner particles to a negative polarity when carrier particles coated with silicone resin are employed, but the polarity control effect of the polarity control agents does not last long enough for practical use. Others are not capable from the beginning of controlling toner particles so as to permit them to be charged to a negative polarity with sufficiently high charges.
Further, the charge quantity distribution range of toner particles containing such polarity control agents is broad and some of the toner particles are inversely charged to a positive polarity with time while in use in development. In the end, the toner particles are deposited on the background of latent electrostatic images to be developed. When the charge quantity distribution range of toner particles becomes too broad, the toner particles do not provide high image quality in a stable manner, since some toner particles with a particular range of charge quantity are apt to be exclusively consumed.